The present invention is directed to thermistor bead type sensors for sensing the level of liquid in a container, in general, and more particularly, to a method and apparatus for detecting a dry/wet state of a dual exposed thermistor bead liquid level sensor in which current is conducted through one thermistor bead at a substantially greater level than through the other thermistor bead, including circuitry for detecting the dry/wet state of the sensor over a wide temperature range from the resulting voltages of both of the thermistor beads.
Sensing liquid level in a container, like fuel in an aircraft fuel tank, for example, has been performed using resistive bead type thermistors which have an inverse temperature coefficient. When current is conducted through a thermistor bead in air, i.e. a dry state, the bead increases in temperature due to self-heating and therefore exhibits a lower resistance to the current. In contrast, when the thermistor bead is submersed in a liquid, like jet fuel, for example, the bead is cooled due to the increased thermal conductivity of the surrounding liquid and its resistance to current conducted therethrough is increased. Current vs. voltage (I/V) characteristics of a typical thermistor bead at various temperatures is shown in the graph of FIG. 1. It is readily observable from the graph of FIG. 1 that the voltage across the thermistor becomes a viable measurement for detecting a wet vs. dry (wet/dry) state of the thermistor bead as the current conducted therethrough becomes greater than 45 milliamps.
For example, if the bead current is fixed at say 45 ma, and a voltage reference level is set at approximately 3.2 then for all temperatures within the range of xe2x88x9254xc2x0 C. to +74xc2x0 C., a bead voltage greater than the reference level indicates a wet state and vice versa. Thus, by passing a constant current equal to or greater than 45 ma through a thermistor bead and lowering the bead from the top of the container, it may be determined at what level in the container the thermistor bead becomes submersed into the liquid by detecting the wet/dry state thereof based on the voltage across the thermistor bead and a fixed reference voltage.
Known interface circuitry which uses the above described method of detecting the dry/wet state of a thermistor bead is shown in the block diagram schematic of FIG. 2. Referring to FIG. 2, a thermistor bead 10 is coupled between a constant current source 12 and a common or ground return. The constant current source 12 is powered by a power supply 14 and is operative to conduct current through the thermistor bead 10. The voltage across the thermistor bead 10 is sensed by one input of a comparator circuit 16 which is also powered by the supply 14 and common return. A fixed reference voltage is generated by a circuit 18 which is powered by the supply 14. The comparator circuit 16 compares the reference voltage which is coupled to another input thereof with the thermistor bead voltage. When the thermistor bead voltage exceeds the reference voltage, a wet bead state is effected at the output of the comparator 16 and when the thermistor bead voltage is less than the reference voltage, a dry bead state is effected at the output of the comparator 16.
As noted above, to insure proper performance of the thermistor bead and detection circuitry using the above described method, the bead 10 should be biased with a constant current equal to or greater than 45 milliamps where voltage levels across the bead are dry/wet distinct for all practical temperature environments ( see the graphs of FIG. 1, for example). For level sensing of combustible liquids in a container with a thermistor bead, the bias current level of 45 milliamps may not be considered safe, and thus unacceptable. For example, the FAA has deemed this bias current level unacceptable in terms of the maximum allowable current that may enter an aircraft fuel tank. Only currents less than 30 milliamps RMS with justification have been deemed acceptable by the FAA for aircraft fuel tanks.
However, as the bead bias current is lowered to less than 30 ma, it is no longer possible to distinguish a wet bead state from a dry bead state based on the comparison of the bead voltage to a fixed reference voltage across a wide operating temperature range, like xe2x88x9254xc2x0 C. to 74xc2x0 C., for example, especially between a dry state at xe2x88x9254xc2x0 C. and a wet state at 74xc2x0 C. Accordingly, at bead currents less than 30 ma, a single thermistor bead may not be an acceptable level measurement sensor for combustible liquids across a wide operating temperature range using interface circuitry implementing the above described traditional method.
In accordance with one aspect of the present invention, a method of detecting a dry/wet state of a dual thermistor bead sensor over a wide operating temperature range comprises the steps of: conducting a first predetermined current through one of the thermistor beads of the sensor; conducting a second predetermined current through the other of the thermistor beads of the sensor, the second predetermined current being substantially less than the first predetermined current; measuring a first voltage across the one thermistor bead in response to the first predetermined current and generating a first signal representative thereof; measuring a second voltage across the other thermistor bead in response to the second predetermined current and generating a second signal representative thereof; modifying the second signal by an offset and gain to generate a third signal; and detecting the dry/wet state of the sensor over the wide operating temperature range based on the first and third signals. The first predetermined current is conducted through the one thermistor bead at an average current which renders the one thermistor bead a sense thermistor bead, and the second predetermined current is conducted through the other thermistor bead at an average current which renders the other thermistor bead a reference thermistor bead.
In accordance with another aspect of the present invention, apparatus for detecting a dry/wet state of a dual thermistor bead sensor over a wide operating temperature range comprises: a first circuit coupled to the sensor for conducting a first predetermined current through one of the thermistor beads of the sensor and for conducting a second predetermined current through the other of the thermistor beads of the sensor, the second predetermined current being substantially less than the first predetermined current; a second circuit coupled to the sensor for measuring a first voltage across the one thermistor bead in response to the first predetermined current and generating a first signal representative thereof; a third circuit coupled to the sensor for measuring a second voltage across the other thermistor bead in response to the second predetermined current and generating a second signal representative thereof; a fourth circuit coupled to the third circuit for modifying the second signal by an offset and gain to generate a third signal; and a fifth circuit for detecting the dry/wet state of the sensor over the wide operating temperature range based on the first and third signals.
In accordance with yet another aspect of the present invention, a method of detecting a dry/wet state of a dual thermistor bead sensor over a wide operating temperature range comprises the steps of: conducting a first predetermined current through one of the thermistor beads of said sensor; conducting a second predetermined current through the other of the thermistor beads of said sensor, said second predetermined current being substantially less than said first predetermined current; measuring a first voltage across said one thermistor bead in response to said first predetermined current and generating a first signal representative thereof; measuring a second voltage across said other thermistor bead in response to said second predetermined current and generating a second signal representative thereof; combining the first and second signals to generate a third signal; generating a reference signal; and detecting the dry/wet state of said sensor over said wide operating temperature range based on a comparison of said third signal to said reference signal.
In accordance with still another aspect of the present invention, apparatus for detecting a dry/wet state of a dual thermistor bead sensor over a wide operating temperature range comprises: a first circuit coupled to said sensor for conducting a first predetermined current through one of the thermistor beads of said sensor and for conducting a second predetermined current through the other of the thermistor beads of said sensor, said second predetermined current being substantially less than said first predetermined current; a second circuit coupled to said sensor for measuring a first voltage across said one thermistor bead in response to said first predetermined current and generating a first signal representative thereof; a third circuit coupled to said sensor for measuring a second voltage across said other thermistor bead in response to said second predetermined current and generating a second signal representative thereof; a fourth circuit coupled to said second and third circuits for combining the first and second signals to generate a third signal; and a fifth circuit for detecting the dry/wet state of said sensor over said wide operating temperature range based on a comparison of said third signal with a reference signal.